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Binding energy per… (Class 12 Physics SQP 2024-25, Q20)

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PhysicsCBSE Class 12 · Sample paper, session 2024-25 · 2 marks · Very short answer

CBSE Class 12 Physics sample paper 2024-25: question 20

From the official CBSE sample question paper for 2024-25, section B, question 20, for 2 marks. Try it first; the hints and our step-by-step solution are below.

Q20 · 2 marks · Very short answerCompetency-basedNuclei

Binding energy per nucleon vs mass number curve for nuclei is shown in the figure. W, X, Y and Z are four nuclei indicated on the curve. Identify which of the following nuclei is most likely to undergo
(i) Nuclear Fission
(ii) Nuclear Fusion.
Justify your answer.

Figure in the paper: Graph of B.E./A (MeV) vs mass number A. Curve rises steeply, peaks near A=60 and then slowly falls. Points: Z at A=30 with 5.0 MeV; Y at A=60 with 8.5 MeV (peak); X at A=90 with 8.0 MeV; W at A=190 with 7.5 MeV. See it in the official paper.

Source: official sample paper from cbseacademic.nic.in (PDF). See it in the full Sample paper 2024-25.

Hints

Open one hint at a time and try again before the next.

Hint 1

Energy is released when the binding energy per nucleon increases.

Hint 2

Which nucleus would move towards the peak of the curve (A ≈ 60) by splitting, and which by combining?

Step-by-step solution

Each step shows the marks it earns, as an examiner would award them. Checked by an independent re-solve from the question paper.

  1. (i) Nuclear fission: nucleus W (A = 190).

    ½ mark

  2. W is a heavy nucleus with a lower binding energy per nucleon (7.5 MeV) than medium nuclei such as X and Y. Splitting it into two medium-mass nuclei raises the B.E./A, so energy is released.

    ½ mark

  3. (ii) Nuclear fusion: nucleus Z (A = 30).

    ½ mark

  4. Z is a light nucleus with a low B.E./A (5.0 MeV). Two such nuclei fusing give a nucleus of A ≈ 60 near the peak (8.5 MeV per nucleon), so the B.E./A increases and energy is released.

    ½ mark

Answer: (i) Fission: W; (ii) Fusion: Z

Where the marks are

  • Identifies W for fission (½ mark)
  • Justifies it: heavy nucleus, lower B.E./A, products have higher B.E./A (½ mark)
  • Identifies Z for fusion (½ mark)
  • Justifies it: light nucleus, low B.E./A, fused product has higher B.E./A (½ mark)

Revise the chapter

Practise this chapterBoard-style questions from Nuclei, one at a time, marked against the answer key and marking points.Back to the full paperSample paper 2024-25: every question, by section.

The question is transcribed from the official paper; figures are described in words. The solution, hints and chapter tag are ours. Spotted an error? Write tosupport@questionbuddy.in.